Interpretive Summary: The placenta is the earliest fetal organ to develop in all eutherian mammals and is essential for intrauterine development. The placenta is entrusted with a number of functions including the transport of nutrients and gases to support development, and to provide protection against maternal immune response during pregnancy. To achieve these diverse functions, in the rat is composed of a number of three diverse layers, composed of distinctive cell types. In the present report, we utilize massively parallel signature sequencing, to examine the gene expression of these layers. Sequencing-based analysis allows to discover transcripts are unprecedented levels of abundance. Our analysis revealed the unique gene networks that orchestrate functions within these functional compartments. Transport and vasculature-related processes predominated in the labyrinth, hormone secretion in the junctional, and immune interactions in the metrial gland. Most importantly, the present dataset of provides a novel resource to understand zonal gene expression and function in the placenta.

Technical Abstract:
The rat placentation site is distinctly organized into interacting zones, the so-called labyrinth, junctional, and metrial gland compartments. These zones house unique cell populations equipped to undertake myriad prescribed functions including transport, hormonal responses, and immune interactions. Although much is known about the genesis of these cell types and specific markers that characterize each zone, a detailed global overview of gene expression in the three zones is absent. In this report, we used massively parallel sequencing (RNA-seq) to assess mRNA expression profiles and generated transcriptomic maps for each zone of the late-gestation rat placentation site (18.5 d post-coitum). Analysis of expression profiles revealed that each compartment expressed a unique signature, characterized by biological processes specific to the zone. Transport and vasculature-related processes predominated in the labyrinth, hormone secretion in the junctional, and immune interactions in the metrial gland. Furthermore, our analysis identified approximately 4000 differentially expressed genes within the zones. Using k-means clustering, we identified transcription factors with highest expression in either labyrinth, junctional, or metrial gland. Direct interaction (pathway) analysis revealed unique transcription factor networks operating in each compartment. The site-specific expression of 27 transcription factors in the three zones was ascertained via quantitative PCR and protein expression of six transcription factors was confirmed by immunohistochemistry. Finally, we elucidated the expression of key developmentally important families (Sox, GATA, Fox, Wnt, Tead, and IGF/IGFBP) in the placentation site to reveal novel expression of these several factors. The present dataset of provides a novel resource to understand zonal gene expression and function in the placenta.